1. Field of the Invention
The invention relates in general to evaluation of breathing problems, and specifically to a non-invasive method and apparatus for evaluating diaphragm function and diaphragm fatigue in patients suffering from emphysema and other pulmonary related dysfunctions.
2. Description of the Related Art
Oxygen enters the human body from air pumped into the lungs during breathing. The lungs expand and contract passively with the volume of a lung cavity formed by a rib cage and a muscle called a diaphragm. When the diaphragm contracts, the volume of the lung cavity increases and the lungs fill with air ("inspiration"). Expansion of the rib cage has the same effect, and normal inspiration involves both contraction of the diaphragm and expansion of the rib cage. The total volume of air inspired during a full inspiration cycle is called "the tidal volume." The relative part of the tidal volume contributed by the contraction of the diaphragm and by the expansion of the rib cage varies between individuals, and may vary with time in the same individual, but contraction of the diaphragm is necessary for effective inspiration.
Expiration starts when the diaphragm relaxes and the rib cage contracts, and ends when the lung cavity has reached minimum volume. The diaphragm can not actively aid in the expiration, it is simply pressed up against the lung cavity by the prevailing pressure in the abdomen when it relaxes. The rib cage also contracts passively when the expansion muscles in the rib cage relax. Extra expiration effort can, however, be contributed by contraction muscles in the rib cage and in the abdomen.
A healthy person feels no strain or fatigue in normal breathing. In order to induce breathing fatigue in a healthy person, the person must be forced to breath through a restricted mouthpiece, which adds a substantial pressure drop in series with the airways.
Known methods for evaluation of diaphragm function in human patients require time-intensive set-ups, and require measurement of the pressure differential across the diaphragm by means of two pressure sensors, which must be swallowed by the patient. The two pressure sensors must be arranged to measure the trans-diaphragmatic pressure and the gastric pressure. The actual evaluation requires time consuming and strenuous breathing exercises by the patient through restricted mouthpieces, while air flow and the pressure differential are measured. A comprehensive description of the known procedure for evaluating diaphragmatic fatigue in healthy persons is given in a publication by F. Bellemare and A. Grassino, "Effect of pressure and timing of contraction on human diaphragm fatigue." Journal of Applied Physiology: Resp. Environ. Exercise Physiol. 53(5): 1190-1195, 1982.
Patients with emphysema have obstructed lungs and airways. The obstructions resist airflow, and cause a pressure drop that must be overcome by the breathing mechanism. In severe cases of emphysema, normal breathing at rest can lead to diaphragmatic fatigue. At that stage, the patient requires breathing assistance. First, he will be given oxygen, which increases the rate of oxygen transfer in the lungs and thereby lowers the tidal volume required per breath, so shallower and less fatigue breathing will satisfy the patient's oxygen need. If this is not sufficient, the patient must be connected to a respirator for assisted breathing.
A respirator provides a constant positive breathing pressure during inspiration, maintaining a suitable inspiration period and a tidal volume that will provide proper oxygenation. During expiration, the respirator maintains an opposite pressure differential, providing a suitable expiration period. The respirator thus compensates for the pressure drops in the obstructed airways. Depending on the amount of pressure provided by the respirator, the work to be performed by the diaphragm and the rib cage in each breathing cycle can either be reduced to a tolerable level, or entirely eliminated. A respirator, however, must be set to provide a sufficient tidal volume, and it is not possible to determine from the pressure in the respirator if the patient takes any active part in the breathing. This is unfortunate, because the muscle tone of the diaphragm will deteriorate with time when breathing is provided passively by a respirator. If the patient remains on the respirator too long, he may eventually lose his ability to breath effectively on his own.
Breathing dysfunction can also be caused by paralysis of the diaphragm and/or the rib cage muscles, for instance after trauma to the brain or to the spinal cord. In such cases, the patient must also be connected to a respirator to survive. Even in such cases it would be helpful to establish if, or when, the patient regains partial or full ability to breathe on his own, without first disconnecting the respirator entirely.
Known methods for evaluating diaphragmatic function by measurement of the pressure differential across the diaphragm by means of pressure transducers swallowed by the patient are not feasible for patients on a respirator. Doctors must accordingly rely on experience to guess when, or if, the respirator could be removed. Even temporary removal of a respirator may cause severe discomfort to the patient, and the doctor would still not be able to determine what degree of breathing ability the patient had achieved.
A publication by J. Wait, P. Nahormek, W. Yost, and D. Rochester: "Diaphragmatic thickness-lung volume relationship in vivo," J. Appl. Physiol. 67(4): 1560-1568, 1989, describes a non-invasive method for measuring the thickness of a human diaphragm based on M-mode ultrasonography, and finds a linear relationship between changes in the diaphragm thickness and the volume of air inspired into the lung. The publication does not discuss evaluation of diaphragmatic function or fatigue.